Pathogenicity in Plasmopara halstedii (Sunflower downy mildew) races 710, 714 and 704

Pathogenicity including virulence and aggressiveness characteristics was studied on three Plasmopara halstedii (the causal agent of downy mildew) isolates of races 710, 714 and 704 using five single zoosporangium isolates per pathogen isolate. Based on the reaction for the P. halstedii isolates to four sunflower hybrids H1 to H4 varying only in their downy mildew resistance genes, there were differences in virulence spectrum in pathogen isolates. Analysis of five single zoosporangium isolates for P. halstedii isolates showed significant variability within pathogen isolate for all aggressiveness criteria. There were no significant differences among pathogen isolates for all aggressiveness criteria. There were significant differences in the morphology of zoosporangia and sporangiophores for pathogen isolates. Genetic relationships were detected between pathogen isolates using 12 EST-derived markers. There was no intra-race genetic variation, but three genetically-identified groups were detected among pathogen isolates